Internet Engineering Task Force MMUSIC WG
Internet Draft M. Handley, H. Schulzrinne, E. Schooler
ietf-mmusic-sip-02.txt ISI/Columbia U./Caltech
March 27, 1997
Expires: September 25, 1997
SIP: Session Initiation Protocol
STATUS OF THIS MEMO
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Distribution of this document is unlimited.
ABSTRACT
Many styles of multimedia conferencing are likely to co-
exist on the Internet, and many of them share the need to
invite users to participate. The Session Initiation
Protocol (SIP) is a simple protocol designed to enable
the invitation of users to participate in such multimedia
sessions. It is not tied to any specific conference
control scheme, providing support for either loosely or
tightly controlled sessions. In particular, it aims to
enable user mobility by relaying and redirecting
invitations to a user's current location.
This document is a product of the Multiparty Multimedia
Session Control (MMUSIC) working group of the Internet
Engineering Task Force. Comments are solicited and
should be addressed to the working group's mailing list
at confctrl@isi.edu and/or the authors.
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Authors' Note
This document is the result of a merger of the Session Invitation
Protocol (draft-ietf-mmusic-sip-00.txt) and the Simple Conference
Invitation Protocol (draft-ietf-mmusic-scip-00.txt), and of an
attempt to make SIP more generic and to fit into a more flexible
infrastructure that includes companion protocols including SDP, HTTP
and RTSP.
Changes
Since version -01, the following things have changed:
o CAPABILITIES to OPTIONS for closer alignment with HTTP and
RTSP;
o Path to Via for closer alignment with HTTP and RTSP;
o Content type meta changed to application, since "meta" doesn't
exist as a top-level Internet media type.
o Formatting closer to HTTP and RTSP.
o Explain relationship to H.323.
1 Introduction
There are two basic ways to locate and to participate in a multimedia
session:
o The session is advertised, users see the advertisement, then
join the session address to participate.
o Users are invited to participate in a session, which may or
may not already be advertised.
The Session Description Protocol (SDP) [1] together with the Session
Announcement Protocol (SAP) [2], provide a mechanism for the former.
This document presents the Session Initiation Protocol (SIP) to
perform the latter. SIP MAY also use SDP to describe a session.
Figure omitted in ASCII version
Figure 1: Session Lifecycle
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We make the design decision that how a user discovers that a session
exists is orthogonal to a session's conference control model. Figure
1 shows a potential place for SIP in the lifecycle of both
lightweight sessions and in more tightly-coupled conferencing. Note
that the Session Initiation Protocol and the Session Announcement
Protocol may be invoked or re-invoked at later stages in a session's
lifecycle.
The Session Initiation Protocol is also intended to be used to invite
servers into sessions. Examples might be where a recording server can
be invited to participate in a live multimedia session to record that
session, or a video-on-demand server can be invited to play a video
stream into a live multimedia conference. In such cases we would like
SIP to lead the server gracefully into the control protocol that
controls the actual recording and playback.
We also make the design decision that inviting a user to participate
in a session is independent of quality of service (QoS) guarantees
for that session. Such QoS guarantees (if they are required) may be
dependent on the full membership of the session, and this may or may
not be known to the agent performing session invitation.
SIP offers some of the same functionality as H.323, but can also be
used in conjunction with it. In this mode, SIP is used to locate the
appropriate terminal, where the terminal is identified by its H.245
address [TBD: what does this look like?]. An H.323-capable terminal
then proceeds with a normal H.323/H.245 invitation [3].
1.1 Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC xxxx [4].
1.2 Terminology
This specification uses a number of terms to refer to the roles
played by participants in SIP communications. The definitions of
client, server and proxy are similar to those used by HTTP.
Client: An application program that establishes connections for the
purpose of sending requests. Clients may or may not interact
directly with a human user.
Initiator: The party initiating a conference invitation. Note that
the calling party does not have to be the same as the one
creating a conference.
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Invitation: A request sent to attempt to contact a user (or service)
to request that they participate in a session.
Invitee, Invited User: The person or service that the calling party
is trying to invite to a conference.
Location server: A program that is contacted by a client and that
returns one or more possible locations for the user or service
without contacting that user or service directly.
Location service: A service used by a location server to obtain
information about a user's possible location.
Proxy, Proxy server: An intermediary program that acts as both a
server and a client for the purpose of making requests on behalf
of other clients. Requests are serviced internally or by passing
them on, possibly after translation, to other servers. A proxy
must interpret, and, if necessary, rewrite a request message
before forwarding it.
Server: An application program that accepts connections in order to
service requests by sending back responses. A server may be the
called user agent, a proxy server, or a location server.
User Agent, Called User Agent: The server application which contacts
the invitee to inform them of the invitation, and to return a
reply.
Any given program may be capable of acting both as a client and a
server. A typical multimedia conference controller would act as a
client to initiate calls or to invite others to conferences and as a
server to accept invitations.
1.3 General Requirements
SIP is a Session Initiation Protocol. It is not a conference control
protocol. SIP can be used to perform a search for a user or service
and to request that that user or service participate in a session.
Once SIP has been used to initiate a multimedia session SIP's task is
finished. There is no concept of a SIP session (as opposed to a SIP
search for a user or service). If whatever conference control
mechanism is used in the session needs to add or remove a media
stream, SIP may be used to perform this task, but again, once the
information has been successfully conveyed to the participants, SIP
is then no longer involved.
SIP must be able to utilize both UDP and TCP as transport protocols.
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From a performance point of view, UDP is preferable as it allows the
application to more carefully control the timing of messages, it
allows parallel searches without requiring connection state for each
outstanding request, and allows the use of multicast.
From a pragmatic point of view, TCP allows easier passage through
existing firewalls, and with appropriate protocol design, allows
common SIP, HTTP and RTSP servers.
When TCP is used, SIP can use either one or more than one connection
to attempt to contact a user or to modify parameters of an existing
session. The concept of a session is not implicitly bound to a TCP
connection, so the initial SIP request and a subsequent SIP request
may use different TCP connections or a single persistent connection
as appropriate.
SIP is text based. This allows easy implementation in languages such
as TCL and Perl, allows easy debugging, and most importantly, makes
SIP flexible and extensible. As SIP is only used for session
initiation, it is believed that the additional overhead of using a
text-based protocol is not significant.
Unlike control protocols, there is minimal shared-state in SIP -- in
a minimal implementation the initiator maintains all the state about
the current attempt to locate and contact a user or service - servers
or proxies can be stateless (although they don't have to be). All the
state needed to get a response back from a server to the initiator is
carried in the SIP request itself - this is also necessary for loop
prevention.
Whilst redesigning SIP, we have attempted to ensure that it
has a clear interaction with the currently evolving Real-
Time Stream Control Protocol.
1.4 Addressing
SIP is a protocol that exchanges messages between peer user agents or
proxies for user agents. We assume the user agent is an application
that acts on behalf of the user it represents (thus it is sometimes
described as a client of the user) and that is co-resident with that
user. A proxy for a user agent serves as a forwarding mechanism or
bridge to the actual location of the user agent. We also refer to
such proxies as location server
In the computer realm, the equivalent of a personal telephone number
combines the user's login id ( mjh ) with a machine host name (
metro.isi.edu ) or numeric network address ( 128.16.64.78 ). A user's
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location-specific address can be obtained out-of-band, can be learned
via existing media agents, can be included in some mailers' message
headers, or can be recorded during previous invitation interactions.
However, users also publish several well-known addresses that are
relatively location-independent, such as email or web home-page
addresses. Rather than require that users provide their specific
network locales, we can take advantage of email and web addresses as
being (relatively) memorable, and also leverage off the Domain Name
Service (DNS) to provide a first stage location mechanism. Note that
an email address ( M.Handley@cs.ucl.ac.uk ) is usually different from
the combination of a specific machine name and login name (
mjh@mercury.lcs.mit.edu ). SIP should allow both forms of addressing
to be used, with the former requiring a location server to locate the
user.
One perceived problem of email addressing is that it is possible to
guess peoples' addresses and thus the system of unlisted (in the
telephone directory) numbers is more of a problem. However, this
really only provides security through obscurity, and real security is
better provided through authentication and call screening.
1.5 Call Setup
Call setup is a multi-phase procedure. In the first phase, the
requesting client tries to ascertain the address where it should
contact the remote user agent or user agent proxy. The local client
checks if the user address is location-specific. If so, then that is
the address used for the remote user agent. If not, the requesting
client looks up the domain part of the user address in the DNS. This
provides one or more records giving IP addresses. If a new service
(SRV) resource record [5] is returned giving a location server, then
that is the address to contact next. If no relevant resource record
is returned, but an A record is returned, then that is the address to
contact next. If neither a resource record or an A record is
returned, but an MX record is returned, then the mail host is the
address to contact next.
Presuming an address for the invitee is found from the DNS, the
second and subsequent phases basically implement a request-response
protocol. A session description (typically using SDP format) is sent
to the contact address with an invitation for the user to join the
session.
This request may be sent over a TCP connection or as a single UDP
datagram (the format of both is the same and is described later), and
is sent to a well-known port.
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If a user agent or conference server is listening on the relevant
port, it can send one of the responses below. If no server or agent
is listening, an ICMP port-unreachable response will be triggered
which should cause the TCP connection setup to fail or cause a UDP
send failure on retransmissions.
1.6 Locating a User
It is expected that a user is situated at one of several frequented
locations. These locations can be dynamically registered with a
location server for a site (for a local area network or
organization), and incoming connections can be routed simultaneously
to all of these locations if so desired. It is entirely up to the
location server whether the server issues proxy requests for the
requesting user, or if the server instructs the client to redirect
the request.
In general a reply MUST be sent by the same mechanism that the
request was sent by. Hence, if a request was unicast, then the reply
MUST be unicast back to the requester; if the request was multicast,
the reply MUST be multicast to the same group to which the request
was sent; if the request was sent by TCP, the reply MUST be sent by
TCP.
In all cases where a request is forwarded onwards, each host relaying
the message SHOULD add its own address to the path of the message so
that the replies can take the same path back, thus ensuring correct
operation through compliant firewalls and loop-free requests. On the
reply path, these routing headers MUST be removed as the reply
retraces the path, so that routing internal to sites is hidden. When
a multicast request is made, first the host making the request, then
the multicast address itself are added to the path.
2 Notational Conventions and Generic Grammar
Since many of the definitions and syntax are identical to HTTP/1.1,
this specification only points to the section where they are defined
rather than copying it. For brevity, [HX.Y] is to be taken to refer
to Section X.Y of the current HTTP/1.1 specification (RFC 2068).
All the mechanisms specified in this document are described in both
prose and an augmented Backus-Naur form (BNF) similar to that used in
RFC 2068 [H2.1]. It is described in detail in [6].
In this draft, we use indented and smaller-type paragraphs to provide
background and motivation.
3 Protocol Parameters
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3.1 SIP Version
applies, with HTTP replaced by SIP.
Applications sending Request or Response messages, as defined by this
specification, MUST include an SIP-Version of "SIP/2.0". Use of this
version number indicates that the sending application is at least
conditionally compliant with this specification.
3.2 UCI: Universal Communication Identifier
[TBD: describe all legal address formats.]
4 SIP Message
All messages are text-based, using the conventions of HTTP/1.1
[H4.1], except for the additional ability of SIP to use UDP. When
sent over TCP or UDP, multiple requests can be carried in a single
TCP connection or UDP datagram. UDP Datagrams should not normally
exceed the path MTU in size if it is known, or 1,000 bytes if the MTU
is unknown.
4.1 Message Types
SIP messages consist of requests from client to server and responses
from server to client.
SIP-message = Request | Response ; HTTP/1.1 messages
Request (section 5) and response (section 6) messages use the generic
message format of RFC 822 for transferring entities (the payload of
the message). Both types of messages consist of a start-line, one or
more header fields (also known as "headers"), an empty line (i.e., a
line with nothing preceding the CRLF) indicating the end of the
header fields, and an optional message-body.
generic-message = start-line
*message-header
CRLF
[ message-body ]
start-line = Request-Line | Status-Line
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In the interest of robustness, servers SHOULD ignore any empty
line(s) received where a Request-Line is expected. In other words, if
the server is reading the protocol stream at the beginning of a
message and receives a CRLF first, it should ignore the CRLF.
4.2 Message Headers
HTTP header fields, which include general-header (section ),
request-header (section ), response-header (section ), fields, follow
the same generic format as that given in Section 3.1 of RFC 822. Each
header field consists of a name followed by a colon (":") and the
field value. Field names are case-insensitive. The field value may be
preceded by any amount of LWS, though a single SP is preferred.
Header fields can be extended over multiple lines by preceding each
extra line with at least one SP or HT. Applications SHOULD follow
"common form" when generating HTTP constructs, since there might
exist some implementations that fail to accept anything beyond the
common forms.
message-header = field-name ":" [ field-value ] CRLF
field-name = token
field-value = *( field-content | LWS )
field-content =
The order in which header fields with differing field names are
received is not significant.
Multiple message-header fields with the same field-name may be
present in a message if and only if the entire field-value for that
header field is defined as a comma-separated list (i.e., #(values) ).
It MUST be possible to combine the multiple header fields into one
"field-name: field-value" pair, without changing the semantics of
the message, by appending each subsequent field-value to the first,
each separated by a comma. The order in which header fields with the
same field-name are received is therefore significant to the
interpretation of the combined field value, and thus a proxy MUST NOT
change the order of these field values when a message is forwarded.
4.3 Message Body
The rules for when a message-body is allowed in a message differ for
requests and responses.
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The presence of a message-body in a request is signaled by the
inclusion of a Content-Length or Transfer-Encoding header field in
the request's message-headers. A message-body MAY be included in a
request only when the request method allows an entity-body.
For response messages, whether or not a message-body is included with
a message is dependent on both the request method and the response
status code (section ). All 1xx (informational) responses MUST NOT
include a message-body. All other responses do include a message-
body, although it may be of zero length.
4.4 Message Length
When a message-body is included with a message, the length of that
body is determined by one of the following (in order of precedence):
1. Any response message which MUST NOT include a message-body
(such as the 1xx responses) is always terminated by the
first empty line after the header fields, regardless of the
entity-header fields present in the message.
2. Otherwise, a Content-Length header MUST be present. (This
requirement differs from HTTP/1.1.) Its value in bytes
represents the length of the message-body.
The "chunked" transfer encoding of HTTP/1.1 MUST NOT be used for SIP.
4.5 General Header Fields
There are a few header fields which have general applicability for
both request and response messages. These header fields apply only to
the message being transmitted.
general-header = Date ; Section
| Transfer-Encoding ; Section
| Via ; Section
General-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields may be given the semantics of general
header fields if all parties in the communication recognize them to
be general-header fields.
5 Request
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The Request-Line begins with a method token, followed by the
Request-URI and the protocol version, and ending with CRLF. The
elements are separated by SP characters. No CR or LF are allowed
except in the final CRLF sequence.
Request-Line = Method SP Request-URI SP SIP-Version CRLF
The method may be either INVITE or CAPABILITY. The request ID may
be any URL encoded string that can be guaranteed to be globally
unique for the duration of the request. Using the initiator's IP-
address, process id, and instance (if more than one request is being
made simultaneously) satisfies this requirement.
6 Response
[H6] applies except that HTTP-Version is replaced by SIP-Version
define some HTTP codes.
After receiving and interpreting a request message, the recipient
responds with an SIP response message.
Response = Status-Line ; Section
*( general-header ; Section
| response-header ; Section
| entity-header ) ; Section
CRLF
[ message-body ] ; Section
6.1 Status-Line
The first line of a Response message is the Status-Line , consisting
of the protocol version followed by a numeric status code, the
sequence number of the corresponding request and the textual phrase
associated with the status code, with each element separated by SP
characters. No CR or LF is allowed except in the final CRLF sequence.
Note that the addition of a
Status-Line = SIP-Version SP Status-Code SP seq-no SP Reason-Phrase CRLF
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6.1.1 Status Code and Reason Phrase
The Status-Code element is a 3-digit integer result code of the
attempt to understand and satisfy the request. These codes are fully
defined in section10. The Reason-Phrase is intended to give a short
textual description of the Status-Code. The Status-Code is intended
for use by automata and the Reason-Phrase is intended for the human
user. The client is not required to examine or display the Reason-
Phrase
The first digit of the Status-Code defines the class of response. The
last two digits do not have any categorization role. There are 5
values for the first digit:
o 1xx: Informational - Request received, continuing process
o 2xx: Success - The action was successfully received,
understood, and accepted
o 3xx: Redirection - Further action must be taken in order to
complete the request
o 4xx: Client Error - The request contains bad syntax or cannot
be fulfilled
o 5xx: Server Error - The server failed to fulfill an apparently
valid request
The individual values of the numeric status codes defined for
SIP/2.0, and an example set of corresponding Reason-Phrase below. The
reason phrases listed here are only recommended -- they may be
replaced by local equivalents without affecting the protocol. Note
that SIP adopts many HTTP/1.1 status codes and adds SIP-specific
status codes in the starting at 450 to avoid conflicts with newly
defined HTTP status codes.
Status-Code = "100" ; Continue
| "200" ; OK
| "300" ; Multiple Choices
| "301" ; Moved Permanently
| "302" ; Moved Temporarily
| "303" ; See Other
| "305" ; Use Proxy
| "400" ; Bad Request
| "401" ; Unauthorized
| "402" ; Payment Required
| "403" ; Forbidden
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| "404" ; Not Found
| "405" ; Method Not Allowed
| "406" ; Not Acceptable
| "407" ; Proxy Authentication Required
| "408" ; Request Time-out
| "409" ; Conflict
| "410" ; Gone
| "411" ; Length Required
| "412" ; Precondition Failed
| "413" ; Request Entity Too Large
| "414" ; Request-URI Too Large
| "415" ; Unsupported Media Type
| "500" ; Internal Server Error
| "501" ; Not Implemented
| "502" ; Bad Gateway
| "503" ; Service Unavailable
| "504" ; Gateway Time-out
| "505" ; HTTP Version not supported
| extension-code
extension-code = 3DIGIT
Reason-Phrase = *
SIP status codes are extensible. SIP applications are not required to
understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, applications MUST
understand the class of any status code, as indicated by the first
digit, and treat any unrecognized response as being equivalent to the
x00 status code of that class, with the exception that an
unrecognized response MUST NOT be cached. For example, if an
unrecognized status code of 431 is received by the client, it can
safely assume that there was something wrong with its request and
treat the response as if it had received a 400 status code. In such
cases, user agents SHOULD present to the user the entity returned
with the response, since that entity is likely to include human-
readable information which will explain the unusual status.
6.1.2 Response Header Fields
The response-header fields allow the request recipient to pass
additional information about the response which cannot be placed in
the Status-Line server and about further access to the resource
identified by the Request-URI
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response-header = Location ; Section
| Proxy-Authenticate ; Section
| Public ; Section
| Retry-After ; Section
| Server ; Section
| Vary ; Section
| WWW-Authenticate ; Section
Response-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields MAY be given the semantics of response-
header fields if all parties in the communication recognize them to
be response-header fields. Unrecognized header fields are treated as
entity-header fields.
7 SIP Message Body
The session description payload gives details of the session the user
is being invited to join. Its Internet media type MUST be given by
the "Content-type:" header field, and the payload length in bytes
MUST be given by the Content-length header field. If the payload has
undergone any encoding (such as compression) then this MUST be
indicated by the Content-encoding: header field, otherwise Content-
encoding: MUST be omitted.
The example below is a request message en route from initiator to
invitee:
INVITE 128.16.64.19/65729 SIP/2.0
Via: SIP/2.0/UDP 239.128.16.254 16
Via: SIP/2.0/UDP 131.215.131.131
Via: SIP/2.0/UDP 128.16.64.19
From: mjh@isi.edu
To: schooler@cs.caltech.edu
Content-type: application/sdp
Content-Length: 187
v=0
o=user1 53655765 2353687637 IN IP4 128.3.4.5
s=Mbone Audio
i=Discussion of Mbone Engineering Issues
e=mbone@somewhere.com
c=IN IP4 224.2.0.1/127
t=0 0
m=audio 3456 RTP/AVP 0
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The first line above states that this is a SIP version 2.0 request.
The via fields give the hosts along the path from invitation
initiator (the first element of the list) towards the invitee. In the
example above, the message was last multicast to the administratively
scoped group 239.128.16.254 with a ttl of 16 from the host
131.215.131.131.
The request header above states that the request was initiated by
mjh@isi.edu (specifically it was initiated from 128.16.64.19, as can
be seen from the Via header) and the user being invited is
schooler@cs.caltech.edu.
In this case, the session description (as stated in the Content-type
header) is a Session Description Protocol (SDP).
The header is terminated by an empty line and is followed by the
session description payload.
If required, the session description can be encrypted using public
key cryptography, and then can also carry private session keys for
the session. If this is the case, four random bytes are added to the
beginning of the session description before encryption and are
removed after decryption but before parsing.
8 Methods
The following methods are defined:
INVITE: The user or service is being invited to participate in the
session. The session description given must be completely
acceptable for a "200 OK" response to be given. This method MUST
be supported by a SIP server.
OPTIONS: The user or service is being queried as to its capabilities.
A server that believes it can contact the user (such as a user
agent where the user is logged in and has been recently active)
MAY respond to this request with a capability set. Support of
this method is OPTIONAL.
Methods that are not supported by a proxy server SHOULD be treated by
that proxy as if they were an INVITE method, and relayed through
unchanged or cause a redirection as appropriate.
Methods that are not supported by a user agent should cause a "501
Not Implemented" response to be returned.
9 Header Field Definitions
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SIP header fields are similar to HTTP header fields in both syntax
and semantics. In general the ordering of the header fields is not of
importance (with the exception of Via fields, see below) but proxies
MUST NOT reorder or otherwise modify header fields other than by
adding a new Via field. This allows an authentication field to be
added after the Via fields that will not be invalidated by proxies.
Field names are not case-sensitive, although their values may be.
Content-Length, Content-Type, To, From header fields are
compulsory. Other fields may be added as required. Header fields MUST
be separated by a single linefeed character. The header MUST be
separated from the payload by an empty line (two linefeed
characters).
A compact form of these header fields is also defined in section 10.9
for use over UDP when the request has to fit into a single packet and
size is an issue.
9.1 Accept
See [H14.1]. This header field is used only for the OPTIONS request
to indicate what description formats are acceptable.
9.2 Accept-Language
See [H14.4]. The Accept-Language request header can be used to allow
the client to indicate to the server in which language it would
prefer to receive reason phrases. This may also be used as a hint by
the proxy as to which destination to connect the call to (e.g., for
selecting a human operator).
9.3 Authentication
Authentication fields provide a digital signature of the remaining
fields for authentication purposes. They are not yet defined The use
of authentication headers is optional. If used, authentication
headers MUST be added to the header after the Via fields and before
the rest of the fields.
HS: Ordering and semantics needs work. Maybe we can recycle
the S/MIME work?
9.4 Confirm
TBD.
9.5 Contact-Host
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TBD.
9.6 From
The request header MUST contain a From request-header field,
indicating the invitation initiator. The field MUST be machine-
usable, as defined my mailbox in RFC 822 (as updated by RFC 1123).
Only a single initiator and a single invited user are allowed to be
specified in a single SIP request.
9.7 Retry-After
The Retry-After response-header field can be used with a 503
(Service Unavailable) response to indicate how long the service is
expected to be unavailable to the requesting client and with a 404
(Not Found) or 451* (Busy) response to indicate when the called party
may be available again. The value of this field can be either an
HTTP-date or an integer number of seconds (in decimal) after the time
of the response.
Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds )
Two examples of its use are
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
9.8 Reason
TBD.
9.9 To
The To request-header field specifies the invited user, with the
same syntax as the From field.
9.10 Via
The Via field indicates the path taken by the request so far. This
prevents request looping and ensures replies take the same path as
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the requests, which assists in firewall traversal and other unusual
routing situations. Initiators MUST add their own Path field to each
request. This Path field MUST be the first field in the request.
Subsequent proxies SHOULD each add their own additional Path field
which MUST be added before any existing Path fields. When a reply
passes through a proxy on the reverse path, that proxies Path field
MUST be removed from the reply.
The format for a Via header is:
Via = "Via" ":" 1#( sent-protocol sent-by [ ttl ] [ comment ] )
sent-protocol = [ protocol-name "/" ] protocol-version
[ "/" transport ]
protocol-name = "SIP" | token
protocol-version = token
transport = "UDP" | "TCP"
sent-by = host [ ":" port ]
ttl = *DIGIT
TTL is included only if the address is a multicast address.
10 Status Code Definitions
The response codes are consistent with, and extend, HTTP/1.1 response
codes. Not all HTTP/1.1 response codes are appropriate, and only
those that are appropriate are given here. Response codes not defined
by HTTP/1.1 are marked with an asterisk, and have codes x50 upwards
to avoid clashes with future HTTP response codes, or 6xx which are
not used by HTTP. The default behavior for unknown response codes is
given for each category of codes.
10.1 Informational 1xx
Informational responses indicate that the server or proxy contacted
is performing some further action and does not yet have a definitive
response. The client SHOULD wait for a further response from the
server, and the server SHOULD send such a response without further
prompting. If UDP transport is being used, the client SHOULD
periodically re-send the request in case the final response is lost.
Typically a server should send a "1xx" response if it expects to take
more than one second to obtain a final reply.
10.1.1 100 Trying
Some further action is being taken (e.g., the request is being
forwarded) but the user has not yet been located.
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10.1.2 150 Ringing
The user agent or conference server has located a possible location
where the user has been recently and is trying to alert them.
10.2 Successful 2xx
The request was successful and MUST terminate a search.
10.2.1 200 OK
The request was successful in contacting the user, and the user has
agreed to participate.
10.3 Redirection 3xx
3xx responses give information about the user's new location, or
about alternative services that may be able to satisfy the call.
They SHOULD terminate an existing search, and MAY cause the initiator
to begin a new search if appropriate.
10.3.1 300 Multiple Choices
The requested resource corresponds to any one of a set of
representations, each with its own specific location, and agent-
driven negotiation information (section 13) is being provided so that
the user (or user agent) can select a preferred representation and
redirect its request to that location.
The response SHOULD include an entity containing a list of resource
characteristics and location(s) from which the user or user agent can
choose the one most appropriate. The entity format is specified by
the media type given in the Content- Type header field. Depending
upon the format and the capabilities of the user agent, selection of
the most appropriate choice may be performed automatically. However,
this specification does not define any standard for such automatic
selection.
If the server has a preferred choice, it SHOULD include the specific
URL for that representation in the Location field; user agents MAY
use the Location field value for automatic redirection.
10.3.2 301 Moved Permanently
The requesting client should retry on the new address given by the
Location: field because the user has permanently moved and the
address this response is in reply to is no longer a current address
for the user. A 301 response MUST NOT suggest any of the hosts in
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the request's path as the user's new location.
10.3.3 302 Moved Temporarily
The requesting client should retry on the new address(es) given by
the Location header. A 302 response MUST NOT suggest any of the hosts
in the request's path as the user's new location.
10.3.4 350* Alternative Service
The call was not successful, but alternative services are possible.
The alternative services are described in the body of the reply.
10.4 Request Failure 4xx
4xx responses are definite failure responses that MUST terminate the
existing search for a user or service. They SHOULD NOT be retried
immediately without modification.
10.4.1 400 Bad Request
The request could not be understood due to malformed syntax.
10.4.2 401 Unauthorized
The request requires user authentication.
10.4.3 402 Payment Required
Reserved for future use.
10.4.4 403 Forbidden
The server understood the request, but is refusing to fulfill it.
Authorization will not help, and the request should not be repeated.
10.4.5 404 Not Found
The server has definitive information that the user does not exist at
the domain specified.
10.4.6 406 Not Acceptable
The user's agent was contacted successfully but some aspects of the
session profile (the requested media, bandwidth, or addressing style)
were not acceptable.
10.4.7 450* Decline
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The user's machine was successfully contacted but the user explicitly
does not wish to participate.
10.4.8 451* Busy
The user's machine was successfully contacted but the user is busy,
or the user does not wish to participate (the ambiguity is
intentional).
10.5 Server Failure 5xx
5xx responses are failure responses given when a server itself has
erred. They are not definitive failures, and SHOULD NOT terminate a
search if other possible locations remain untried.
10.5.1 500 Server Internal Error
The server encountered an unexpected condition that prevented it from
fulfilling the request.
10.5.2 501 Not implemented
The server does not support the functionality required to fulfill the
request. This is the appropriate response when the server does not
recognize the request method and is not capable of supporting it for
any user.
10.5.3 503 Service Unavailable
The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server. The implication
is that this is a temporary condition which will be alleviated after
some delay. If known, the length of the delay may be indicated in a
Retry-After header. If no Retry-After is given, the client SHOULD
handle the response as it would for a 500 response.
Note: The existence of the 503 status code does not imply that a
server must use it when becoming overloaded. Some servers may wish to
simply refuse the connection.
10.6 Search Responses 6xx
6xx responses are failure responses given whilst trying to locate the
specified user or service. They are not definitive failures, and
SHOULD NOT terminate the search if other possible locations remain
untried.
10.6.1 600* Search Failure
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The user agent or proxy server understood the user's address, but the
request was unsuccessful in contacting the user. A proxy might return
this error towards the initiator if an attempt to contact a server
failed for an unknown reason.
10.6.2 601* Not known here
The call was unsuccessful because the user or service was not known
at the address called. This is not a definitive failure; the address
may be valid at another server.
10.6.3 602* Not currently here
The call was unsuccessful because although the the user or service
was known at the address called, the user or service is not currently
located at this address. This is not a definitive failure; the user
may be contactable at another server.
10.6.4 603* Alternative Address
The call was unsuccessful because the user or service is not
available at this location, but one or more alternative non-
definitive locations are suggested to try in addition to any that may
already be being tried. A 603 response MUST NOT suggest any of the
hosts in the request's path as an alternative location.
10.7 Example: Normal Replies
An example reply is given below. The first line of the reply states
the SIP version number, that it is a "200 OK" reply, which means the
request was successful. The Via header are taken from the request,
and entries are removed hop by hop as the reply retraces the
request's path. A new authentication field is added by the invited
user's agent if required. The session ID is taken directly from the
original request, along with the request header. The original sense
of From field is preserved (i.e, it's the session originator).
In addition, a Contact-host field is added giving details of the
host the user was located on, or alternatively the relevant proxy
contact point which should be reachable from the invitation
initiator's host.
SIP/2.0 200 128.16.64.19/65729
Via: SIP/2.0/UDP 239.128.16.254 16
Via: SIP/2.0/UDP 131.215.131.131
Via: SIP/2.0/UDP 128.16.64.19 1
From: mjh@isi.edu
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To: schooler@cs.caltech.edu
Contact-host: 131.215.131.147
This same format is used for replies for other categories of reply,
except that some of then may require payloads to be carried.
If the invited user's agent requires confirmation of receipt of a
"200 OK" reply, it may optionally add an additional Confirm: required
header to the body of the message specifying that an acknowledgment
is required. This is only permitted with category 2xx replies. An
example is:
SIP/2.0 200 128.16.64.19/65729
Via: SIP/2.0/UDP 239.128.16.254 16
Via: SIP/2.0/UDP 131.215.131.131
Via: SIP/2.0/UDP 128.16.64.19
From: mjh@isi.edu
To: schooler@cs.caltech.edu
Contact-host: 131.215.131.147
Confirm: required
In response to such a request, the invitation initiators agent should
retransmit its request with an additional Confirm header, with the
value "true" or "false" stating whether the session still exists or
no longer exists respectively (see section 7.1 for details). An
example of an confirmation request is:
INVITE 128.16.64.19/65729 SIP/2.0
Via: SIP/2.0/UDP 239.128.16.254:70 16
Via: SIP/2.0/UDP 131.215.131.131
Via: SIP/2.0/UDP 128.16.64.19
From: mjh@isi.edu
To: schooler@cs.caltech.edu
Confirm: true
Content-type: application/sdp
Content-Length: 187
v=0
o=user1 2353655765 2353687637 IN IP4 128.3.4.5
s=Mbone Audio
i=Discussion of Mbone Engineering Issues
e=mbone@somewhere.com
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c=IN IP4 224.2.0.1/127
t=0 0
m=audio 3456 RTP/AVP 0
Such confirmations are still useful when TCP transport is used as
they provide application level confirmation rather than transport
level confirmation. If they are not used, it is possible that a "200
OK" response may be received after the application making the call
has timed out the call and exited.
10.7.1 Redirects
"603 alternative address" replies and 301 and 302 moved replies
should specify another location using the Location field.
An example of a "603 alternative address" reply is:
SIP/2.0 603 128.16.64.19/65729
Via: SIP/2.0/UDP 131.215.131.131 1
Via: SIP/2.0/UDP 128.16.64.19
From: mjh@isi.edu
To: schooler@cs.caltech.edu
Location: 239.128.16.254 16
Content-length:0
In this example, the proxy (131.215.131.131) is being advised to
contact the multicast group 239.128.16.254 with a ttl of 16. In
normal situations a server would not suggest a redirect to a local
multicast group unless (as in the above situation) it knows that the
previous proxy or client is within the scope of the local group.
For unicast 603 redirects, a proxy MAY query the suggested location
itself or send MAY the redirect on back towards the client. For
multicast 603 redirects, a proxy SHOULD query the multicast address
itself rather than sending the redirect back towards the client as
multicast may be scoped and this allows a proxy within the
appropriate scope regions to make the query.
For 301 or 302 redirects, a proxy SHOULD send the redirect on back
towards the client and terminate any other searches it is performing
for the same request. Multicast 301 or 302 redirects MUST NOT be
generated.
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10.8 Alternative Services
An example of an "350 Alternative Service" reply is:
SIP/2.0 350 128.16.64.19/32492/2
Via: SIP/2.0/UDP 131.215.131.131
Via: SIP/2.0/UDP 128.16.64.19
From: mjh@isi.edu
To: schooler@cs.caltech.edu
Contact-host: IN IP4 131.215.131.131
Content-type: application/sdp
Content-length: 146
v=0
o=mm-server 2523535 0 IN IP4 131.215.131.131
s=Answering Machine
i=Leave an audio message
c=IN IP4 128.16.64.19
t=0 0
m=audio 12345 RTP/AVP 0
In this case, the answering server provides a session description
that describes an "answering machine". If the invitation initiator
decides to take advantage of this service, it should send an
invitation request to the contact host (131.215.131.131) with the
session description provided. This request should contain a different
session id from the one in the original request. An example would
be:
INVITE 128.16.64.19/32492/3 SIP/2.0
Via: SIP/2.0/UDP 128.16.64.19
From: mjh@isi.edu
To: schooler@cs.caltech.edu
Content-type: application/sdp
Content-length: 146
v=0
o=mm-server 2523535 0 IN IP4 128.16.5.31
s=Answering Machine
i=Leave an audio message
c=IN IP4 128.16.64.19
t=0 0
m=audio 12345 RTP/AVP PCMU
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Invitation initiators can choose to treat a "350 Alternative Service"
reply as a failure if they wish to do so.
10.8.1 Negotiation
A "406 Not Acceptable" reply means that the user wishes to
communicate, but cannot support the session described adequately. The
"406 Not Acceptable" reply contains a list of reasons why the session
described cannot be supported. These reasons can be one or more of:
406.1 Insufficient Bandwidth: the bandwidth specified in the session
description or defined by the media exceeds that known to be
available.
406.2 Incompatible Protocol: one or more protocols described in the
request is not available.
406.3 Incompatible Format: one or more media formats described in the
request is not available.
406.4 Multicast not available: the site where the user is located
does not support multicast.
406.5 Unicast not available: the site where the user is located does
not support unicast communication (usually due to the presence
of a firewall).
Other reasons are likely to be added later. It is hoped that
negotiation will not frequently be needed, and when a new user is
being invited to join a pre-existing lightweight session, negotiation
may not be possible. If is up to the invitation initiator to decide
whether or not to act on a "406 Not Acceptable" reply.
A complex example of a "406 Not Acceptable" reply is:
SIP/2.0 406 128.16.64.19/32492/5
From: mjh@isi.edu
To: schooler@cs.caltech.edu
Contact-host: 131.215.131.131
Reason: 406.1, 406.3, 406.4
Content-Type: meta/sdp
Content-Length: 50
v=0
s=Lets talk
b=CT:128
c=IN IP4 131.215.131.131
m=audio 3456 RTP/AVP 7 0 13
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m=video 2232 RTP/AVP 31
In this example, the original request specified 256 kb/s total
bandwidth, and the reply states that only 128 kb/s is available. The
original request specified GSM audio, H.261 video, and WB whiteboard.
The audio coding and whiteboard are not available, but the reply
states that DVI, PCM or LPC audio could be supported in order of
preference. The reply also states that multicast is not available.
In such a case, it might be appropriate to set up a transcoding
gateway and re-invite the user.
Invitation initiators MAY choose to treat "406 Not Acceptable"
replies as a failure if they wish to do so.
10.9 Compact Form
When SIP is carried over UDP with authentication and a complex
session description, it may be possible that the size of a request or
reply is larger than the MTU (or default 1,000-byte limit if the MTU
is not known). To reduce this problem, a more compact form of SIP is
also defined by using alternative names for common header fields.
These short forms are NOT abbreviations, they are field names. No
other abbreviations are allowed.
short field name long field name note
a Confirm from "acknowledge"
c Content-Type
e Content-Encoding
f From
h Contact-Host
l Content-Length
m Location from "moved"
r Reason
t To
v Via
Thus the header in section ?? could also be written:
INVITE 128.16.64.19/65729 SIP/2.0
p:IN IP4 UDP 239.128.16.254 1 16
p:IN IP4 UDP 131.215.131.131 1
p:IN IP4 UDP 128.16.64.19 1
f:mjh@isi.edu
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t:schooler@cs.caltech.edu
c:application/sdp
l:187
v=0
o=user1 53655765 2353687637 IN IP4 128.3.4.5
s=Mbone Audio
i=Discussion of Mbone Engineering Issues
e=mbone@somewhere.com
c=IN IP4 224.2.0.1/127
t=0 0
m=audio 3456 RTP/AVP 0
Mixing short field names and long field names is allowed, but not
recommended. Servers MUST accept both short and long field names for
requests. Proxies MUST NOT translate a request between short and long
forms if authentication fields are present.
11 SIP Transport
SIP is defined so it can use either UDP or TCP as a transport
protocol.
UDP has advantages over TCP from a performance point of view, as the
SIP application can keep control of the precise timing of
retransmissions, and can also make simultaneous call attempts to many
potential locations of many users without needing to keep TCP
connection state for each connection.
TCP has the advantage that clients are simpler to implement because
no retransmission timing code needs to be written and also that it is
possible to have a single server serving SIP and HTTP with very
little extra code.
With UDP, all the additional reliability code is in the client. It is
recommended that servers SHOULD implement both TCP and UDP
functionality as the additional server code required is very small.
Clients MAY implement either TCP or UDP transport or both as they see
fit.
11.1 Reliability using UDP transport
The Session Invitation Protocol is straightforward in operation. Only
the initiating client needs to keep any state regarding the current
connection attempt. SIP assumes no additional reliability from IP.
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Requests or replies may be lost. A SIP client SHOULD simply
retransmit a SIP request until it receives a reply, or until it has
reached some maximum number of timeouts and retransmissions. If the
reply is merely a 1xx Informational progress report, the initiating
client SHOULD still continue retransmitting the request, albeit less
frequently.
When the remote user agent or server sends a final 2xx or 4xx
response (not a 1xx report), it cannot be sure the client has
received the response, and thus SHOULD cache the results until a
connection setup timeout has occurred to avoid having to contact the
user again. The server MAY also choose to cache 3xx or 6xx responses
if the cost of obtaining the response outweighs the cost of caching
it.
It is possible that a user can be invited successfully, but that the
reply that the user was successfully contacted may not reach the
invitation initiator. If the session still exists but the initiator
gives up on including the user, the contacted user has sufficient
information to be able to join the session. However, if the session
no longer exists because the invitation initiator "hung up" before
the reply arrived and the session was to be two-way, the conferencing
system should be prepared to deal with this circumstance.
One solution is for the initiator to acknowledge the invitee's "200
OK" reply. Although not required, in the case of a successful
invitation the invited user's agent can make a confirmation request
in its "200 OK" reply. In this case the initiator's agent sends a
single request with a reply Confirm: true if the request was still
valid or a reply Confirm: false if it was not so that a premature
hang-up can be detected without a long timeout. Such a confirmation
request may be retransmitted by the invited user's agent if it so
desired. Confirmation requests can only be made with "200 OK"
replies, and only the invitation initiator's agent may issue the
actual confirmation.
Only a "200 OK" reply warrants such a confirmation handshake, because
it is the only situation where user-relevant state may be
instantiated anywhere other than at the initiator's client. In all
other cases, it is not necessary that state is maintained. In
particular, when a server makes multiple proxy requests, "5xx Server
Error" and "6xx Search Response" replies do not immediately get
passed back to the invitation initiator, and so no end-to-end
acknowledgment of a failed request is possible.
11.2 Reliability using TCP transport
TCP is a reliable transport protocol, and so we do not need to define
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additional reliability mechanisms. However, we must define rules for
connection closedown under normal operation.
The normal mode of operation is for the client (or proxy acting as a
client) to make a TCP connection to the well-known port of a host
housing a SIP server. The client then sends the SIP request to the
server over this connection and waits for one or more replies. The
client MAY close the connection at any time.
The server MAY send one or more 1xx Informational responses before
sending a single 2xx, 3xx, 4xx, 5xx or 6xx reply. The server MUST NOT
send more than one reply, with the exception of 1xx responses. The
server SHOULD NOT close the TCP connection until it has sent its
final response, at which point it MAY close the TCP connection if it
wishes to. However, normally it is the client's responsibility to
close the connection.
If the server leaves the connection open, and if the client so
desires it may re-use the connection for further SIP requests or for
requests from the same family of protocols (such as HTTP or stream
control commands).
The same application-level confirmation rules apply for TCP as for
UDP.
12 Searching
A basic assumption of SIP is that a location server at the user's
home site either knows where the user resides, knows how to locate
the user, or at the very least knows another location server that
possibly might have a better idea. How these servers get this
information is outside the scope of SIP itself, but it is expected
that many different user-location services will exist for some time.
SIP is designed so that it does not care which location service SIP
servers actually employ.
12.1 Proxy servers: Relaying and Redirection
If a proxy server receives a request for a user whose location it
does not know, and for whom it has no better idea where the user
might be, then the server should return a "601 Not Currently Here"
reply message.
If the server does have an idea how to contact the user, it can
either forward (relay) the request itself, or can redirect the
invitation initiator to another client that is more likely to know by
sending a 603, 301 or 302 response as appropriate. It can also
gateway the request into some other form if some other invitation
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protocol is in use in a region containing the invited user, though in
doing so the server is likely to give up being stateless.
Whether to relay the request or to redirect the request is up to the
server itself. For example, if the server is on a firewall machine,
then it will probably have to relay the request to servers inside the
firewall. Additionally, if a local multicast group is to be used for
user location, then the server is likely to relay the request.
However, if the user is currently away from home, relaying the
request makes little sense, and the server is more likely (though not
compelled) to send a redirect reply. SIP is policy-free on this
issue. In general, local searches are likely to be better performed
by relaying whereas wide-area searches are likely to be better
performed by redirection.
When SIP uses UDP transport, clients and servers can make multiple
simultaneous requests to locate a particular user at low cost. This
greatly speeds up any search for the user, and in most cases will
only result in one successful response. Although several simultaneous
paths may reach the same host, successful responses arriving from
multiple paths will not confuse the client as they should all contain
the same successful host address. However, this does imply that paths
with many levels of relaying should be strongly discouraged as if the
request is fanned out at each hop and relayed many times, request
implosions could result. Thus servers that are not the first hop
servers in a chain of servers SHOULD NOT make multiple parallel
requests, but should send a redirection response with multiple
alternatives. Thus a firewall host can still perform a parallel
search but can control the fanout of the search.
12.2 Parallel Searches: Initiator Behavior
The session initiator may make a parallel search for a user. This can
occur when DNS resolution results in multiple addresses, or when
contacting a remote server results in a "603 Alternative Address"
response containing multiple addresses to try. All such parallel
searches for the same SIP request MUST contain the same SIP Id,
though the sequence number (given in the Path field) SHOULD be
different for each of the parallel searches.
Whilst performing a parallel search, different responses may result
from different servers, and it is important for the initiating client
to handle these responses correctly. In general, the following rules
apply:
o If a 2xx response is received, the invitation was successful,
the user should be informed and all pending requests should be
terminated and/or ignored.
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o If a 4xx response is received the invitation has definitively
failed, the user should be informed, and all pending requests
should be terminated and/or ignored.
o If a 3xx response is received, the search should be terminated
and all pending requests should be terminated and/or ignored.
However, further action MAY be taken depending on the actual
reply without informing the user or alternatively the
invitation MAY be regarded as having failed in which case the
user MUST be informed.
o If a 5xx or 6xx response is received, the particular server
responding is removed from the parallel search and the search
continues. If a "603 Alternative Address" response is
received, the search may be expanded to include those servers
listed in the response that have not already responded. The
user SHOULD NOT be informed unless there are no other servers
left to try, in which case the user MUST be informed.
o If a 1xx response is received, the search continues. The user
MAY be informed as deemed appropriate.
12.3 Parallel Searches: Proxy Behavior
In the same way that an Initiating Client can discover multiple
addresses to try, a proxy server can also discover multiple addresses
that it may try. For a proxy server to be stateless, it must not make
multiple SIP requests because it would then be possible to return a
5xx or 6xx response to the Initiating Client and afterwards obtain a
definitive answer. To be able to make multiple parallel SIP requests,
it must keep state as to the replies it has already received and MUST
NOT return any reply other than 1xx informational replies until it
has received a definitive reply or has no further addresses to try.
Thus faced with DNS resolution giving multiple addresses, a proxy
server that wishes to be stateless should only send a SIP request to
the first address. Similarly a stateless proxy should not attempt to
send SIP request to multiple addresses given in a "603 Alternative
Address" response that is returned it it, but should forward such a
response back towards the initiator.
Proxies that wish to keep state should follow the following rules
regarding responses obtained during a parallel search:
o If a 2xx response is received, the invitation was successful,
the 2xx response should be forwarded back towards the
initiator, and all pending requests should be terminated and/or
ignored.
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o If a 4xx response is received the invitation has definitively
failed, the 4xx response should be forwarded back towards the
initiator, and all pending requests should be terminated and/or
ignored.
o If a 3xx response is received the invitation is regarded by
the proxy as having failed, the 3xx response should be
forwarded back towards the initiator, the search should be
terminated and all pending requests should be terminated and/or
ignored.
o If a 5xx or 6xx response is received, the particular server
responding is removed from the parallel search and the search
continues. If a "603 Alternative Address" response is
received, the search may be expanded to include those servers
listed in the response that have not already responded. No
response other than a periodic "100 Trying" response should be
send towards the initiator unless there are no other servers
left to try, in which case a response SHOULD be sent as
described below.
o If a 1xx response is received, the search continues. The 1xx
response MAY be forwarded towards the initiator as appropriate.
If a proxy had exhausted its search and still not obtained a
definitive response (it received only 1xx, 5xx, and 6xx responses)
the proxy should cache these responses and return the first response
from the following ordered list:
1. 503 Service Unavailable;
2. 500 Server Internal Error;
3. 501 Not Implemented;
4. any other 5xx error not yet defined;
5. 600 Search Failure;
6. 602 Not Currently Here;
7. 601 Not Known Here;
8. any other 6xx error response not yet defined.
If a proxy has exhausted its search and the only response it has
received has been "603 Alternative Address", then the proxy should
send a "600 Search Failure" response if any connection attempt timed
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out or failed, or it should send "602 Not Currently Here" if two or
more "603 Alternative Address" responses only provide references to
each other.
12.4 Change of Transport at a Proxy
Editors note: this section is still incomplete. Several
options exist for where the responsibility should lie for
retransmissions from proxies between TCP and UDP transport.
This section generally assumes local retransmission, but
end-to-end transmission through a chain of proxies is also
possible.
It is possible that a proxy server will receiver a request using TCP
and relay it onwards using UDP or vice-versa. SIP does not assume
end-to-end reliability even when the initiating client is using TCP,
but a SIP client sending a request over TCP MAY assume that the
request has been received by the server it sent the request to.
Retransmission of the request is then not the responsibility of the
client. However, a called user agent SHOULD NOT assume that a 2xx
success response has been received by the invitation initiator, even
if all the path fields in the request indicated TCP transport because
it cannot be certain all those TCP connections still exist. If the
called user agent requires knowledge that the response did reach the
invitation initiator, it MAY add a Confirm: required field to the
reply as it would if the response was sent using UDP.
In the following, the term "TCP-UDP proxy" is used to mean a proxy
that received a request using TCP and relayed it using UDP. Similarly
a "TCP-UDP proxy" receives a reply using UDP and should relay it
using TCP.
12.4.1 Retransmission from a TCP-UDP Proxy
A proxy receiving a request with TCP transport and forwarding that
request using UDP becomes responsible for retransmission of the
request as required and for timing out the request if no answer is
forthcoming.
12.4.2 Retransmissions arriving at a UDP-TCP Proxy
A proxy receiving a request using UDP transport and forwarding that
request using TCP transport may have have SIP request state
associated with that TCP connection or SIP response state associated
with it.
If such a proxy receives a retransmission of the UDP request whilst
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in the state or awaiting a response (i.e, has request state), it
SHOULD NOT forward the duplicate request into the TCP connection
unless the request has been modified, but instead SHOULD respond with
a "100 Trying" response sent back towards the initiator.
Note: This behavior is different from a UDP-UDP proxy which MUST
forward the retransmitted request and MAY additionally respond with a
"100 Trying" response sent back towards the initiator.
If such a proxy receives a retransmission of the UDP request in
response state (i.e, it has already sent a definitive response) then
the proxy MAY retransmit that response if it has cached it.
Alternatively if it has not cached the response, it SHOULD resend the
request towards the called user agent, either via an existing TCP
connection if there is one or via a new TCP connection if there is
not, to obtain a retransmission of the response. In the latter case,
the proxy MAY additionally respond with a "100 Trying" response sent
back towards the initiator.
Note: This behavior is the same as a UDP-to-UDP proxy in the same
circumstances.
12.4.3 Confirmation arriving at a TCP-UDP Proxy
One possible event that may occur is that whilst performing a search
using UDP, a response may arrive that should be relayed back towards
the initiator using TCP, but the TCP connection has been terminated
by the initiator. In this case the proxy MUST NOT attempt to relay
the response (by opening a TCP connection) and should terminate any
outstanding search. In this circumstance only, if the response was a
"200 OK" response with a Confirm: required field, the proxy MAY
resend the request to the Contact Host with a Confirm: false field
to speed hang-up discovery at the called user agent.
12.4.4 Confirmation sent from a UDP-TCP Proxy
Normally a response that arrives at a proxy using TCP that should be
sent back towards the initiator using UDP should be sent once, and
should only be resent if the request is resent from the UDP proxy
closer to the initiator. However, this does not allow for reliable
confirmation.
13 Using Variants for Terminal Negotiation
Redirection allows the called party to indicate several communication
alternatives to the caller using the 300 (Multiple Choices) response,
all reachable using a single published communication identifier.
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The Alternates header in the response contains the variant list.
The response may contain an entity, typically of content type
text/html, providing guidance to the user. The calling user agent is
free to ignore this part and solely rely on the Alternates header.
SIP/2.0 300 Multiple Choices
Date: Thu, 06 Mar 1997 10:08:55 GMT
Alternates:
{"hgs@erlang.cs.columbia.edu" 0.9 {mobility fixed} {class business}
{service IP, voice-mail} {media all} {duplex full}},
{"+12129397042" 0.8 {mobility fixed} {class business}
{service POTS} {media audio} {duplex full}},
{"+12129397000" 0.7 {mobility fixed} {class business}
{service ISDN, attendant} {media audio} {duplex full}
{language en, es, iw}},
{"+12125551212" 0.6 {mobility mobile} {class personal}
{service POTS} {media audio} {duplex full}}
}
Content-Type: text/html
Content-Length: 283
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